1. Field of the Invention
The present invention relates to a wavelength conversion device including a fundamental wave light source and an external resonator, and a wavelength conversion method.
2. Description of the Related Art
A wavelength conversion device including a fundamental wave light source and an external resonator having a nonlinear optical crystal is generally used as a laser light source that oscillates light of a short wavelength laser such as an ultraviolet laser.
FIG. 1 is a schematic configuration diagram of an example of a wavelength conversion device. The wavelength conversion device includes a fundamental wave light source 101 and an external resonator 120. The external resonator 120 includes an input coupling element 121 having a mirror with a predetermined transmittance, mirrors 122 to 124, and a nonlinear optical crystal 125 placed between the input coupling element 121 and the mirror 122. Fundamental wave light for excitation Le emitted from the fundamental wave light source 101 is employed in the external resonator 120 from the input coupling element 121, and is emitted as second harmonic light Lo, for example, to the outside from the mirror 122 having a predetermined transmittance, for example.
Impedance matching is performed as a main method to determine a reflectance (transmittance) of the input coupling element 121 of the related art. In this method, an internal loss of the external resonator is first figured out, and a loss of the input coupling element is determined in order to match with the obtained internal loss. In this case, wavelength conversion efficiency is maximized by selecting a reflectance of the input coupling element so that impedance is matched with the internal loss.
By contrast, U.S. Pat. No. 5,027,361 discloses a method of determining a reflectance of an input coupling element by not only performing impedance matching but also taking a nonlinear loss into consideration.